The primordial binary population. II.

Abstract

We characterize the binary population in the young and nearby OB association Scorpius OB2 (Sco OB2) using available observations of visual, spectroscopic, and astrometric binaries with intermediate-mass primaries. We take into account observational biases by comparing the observations with simulated observations of model associations. The available data indicate a large binary fraction (> 70% with 3σ confidence), with a large probability that all intermediate mass stars in Sco OB2 are part of a binary system. The binary systems have a mass ratio distribution of the form f_q(q) ∝ qγ_q, with γq ≈ -0.4. Sco OB2 has a semi-major axis distribution of the form f_a(a) ∝ aγ_a with γa ≈ -1.0 (Öpik's law), in the range 5 {R}_⊙⪉ a ⪉ 5× 106 {R}_⊙. The log-normal period distribution of Duquennoy & Mayor [1991, A&A, 248, 485] results in too few spectroscopic binaries, even if the model binary fraction is 100%. Sco OB2 is a young association with a low stellar density; its current population is expected to be very similar to the primordial population. The fact that practically all stars in Sco OB2 are part of a binary (or multiple) system demonstrates that multiplicity is a fundamental factor in the star formation process, at least for intermediate mass stars. Appendix A is only available in electronic form at http://www.aanda.org